The IEEE 802.15.4 WPAN (wireless personal area network) standard is defined with respect to a close range (or short distance) communication technology for providing convenience in the interconnection between personal mobile devices (or user equipments).
In the 868/915 MHz band, the IEEE 802.15.4 standard uses BPSK (binary phase-shift keying) so as to provide transmission rates of 20 Kbps and 40 Kbps, and, in the 2.45 GHz band, the IEEE 802.15.4 standard uses O-QPSK (offset quadrature phase-shift keying) so as to provide a transmission rate of 250 Kbps.
FIG. 1 illustrates an exemplary Network Topology based upon IEEE 802.15.4.
In the IEEE 802.15.4 network, two different types of devices, such as FFD (Full Function Device) and RFD (Reduced Function Device), may be involved. The FFD performs functions, such as network initialization, node management, node information storage, and so on, and, herein, the FFD is also referred to as a PAN coordinator (Personal Area Network coordinator), so that the remaining devices can configure any one of the respective networks.
As a device that can perform coordinator functions, the FFD may configure diverse forms (or types) of network topology, and the FFD may perform communication with both FFDs and RFDs. Moreover, since the FFD consumes a relatively larger amount of power in order to perform the coordinator functions, the FFD is generally supplied with power via wired connection.
Conversely, as a device that cannot perform the functions of a coordinator, the RFD becomes the coordinating target of the FFD. More specifically, the RFD may perform communication only with the FFD, and, by assigning the FFD with all network functions, the RFD may use only a minimum stack structure size, thereby being capable of saving (or economizing) operation (or calculation)/memory resource. Accordingly, after locating the PAN coordinator and transmitting data, since the RFD may immediately disconnect its connection, so as to enter (or shift to) a Save (Sleep) mode, the consumed amount of power becomes very small, thereby allowing the RFD to operate on battery power for a long period of time.
Referring to FIG. 1, a device being indicated as “F” represents the FFD, a device being indicated as “R” represents the RFD, and a device being indicated as “P” represents an FFD performing the roles of a coordinator.
FIG. 1 shows two different types of network topology that can be configured by the IEEE 802.15.4 system. Herein, (a) of FIG. 1 shows an example of a star type network (or star topology), and (b) of FIG. 1 shows an example of a Peer to Peer type network (or Peer to Peer topology).
In the star topology, only the communication between the device and the PAN coordinator may be established. At this point, the devices may correspond to a starting point or an ending point of the communication, whereas the PAN coordinator may correspond to a starting point, an ending point, or a router.
In the peer-to-peer topology, each device may perform communication with any one of the devices existing in the network. Therefore, a network having a more complicated structure, such as a mesh network, may be configured.
The star network may operate the devices so that the durability of the battery may be maintained for a long period of time, and, since the Peer to Peer network may configure at least one or more data delivery paths, the Peer to Peer network may have higher data reliability and higher access recognition rate.
FIG. 2 illustrates a structural view of a protocol stack in an IEEE 802.15.4 system.
As it can be known with reference to FIG. 2, the IEEE 802.15.4 protocol stack may consist of a PHY layer (Physical layer) (110), a MAC layer (Medium Access Control layer) (120), and an Upper layer (130).
The PHY layer (110) includes an RF transceiver (or transmitter/receiver) and a related control mechanism, the MAC layer (120) provides access to a physical channel for data transmission.
The Upper Layer (130) is configured of a Network Layer and an Application Layer. The network layer provides functions, such as network configuration, processing, message routing, and so on. The application layer provides the functions targeted by the device. For example, depending upon the type of the program installed therein. i.e., depending upon the type of the program processing the data of the application layer, an IEEE 802.15.4 device (100) may perform as an RFD (Reduced Function Device), an FFD (Full Function Device), or a coordinator.
FIG. 3 illustrates a structure of a super frame being used in the IEEE 802.15.4 system.
In accordance with its low power request, the IEEE 802.15.4 system is configured of an active period and an inactive period. And, the repetition cycle of the active period and the inactive period is referred to as a duty cycle.
The active period is configured of a Beacon, a CAP (Contention Access Period), and CFP (Contention Free Period), and data transmission occurs during the CAP period.
The CFP period is configured of multiple GTSs (Guaranteed Time Slots), and each GTS is allocated to a specific device, so that each device may use the allocated GTS for transmitting and receiving data to and from the PAN coordinator. The GTS may support one PAN and up to a maximum of 7 PANs.
The allocated content of each GTS is determined (or set up) in the form of a GTS descriptor by the PAN coordinator. The GTS descriptors are included in the GTS field of the beacon and are transmitted by the PAN coordinator.
In the GTS allocation method of the conventional IEEE 802.15.4 system, once a device is allocated with a GTS, the corresponding device is continuously allocated with a GTS for each super frame until de-allocation of the GTS is performed upon request of the device or due to a GTS expiration.
However, in this method, since the required GTS may differ for each GTS, this method is inefficient. This is because once a device is allocated with a GTS, no other device can be allocated with the corresponding GTS until the corresponding GTS is de-allocated.
Therefore, a method allowing a GTS to be allocated to each device at different cycle periods and allowing the device to use the GTS by using a time-division method is being required.